A plant pays a fungus in sugars and fats, and the fungus pays back with nutrients, mainly phosphorus. The deal runs in the roots of around 70 percent of land plant species.
The fungi involved are arbuscular mycorrhizal fungi. They cannot make their own food, so they depend on carbon from the plant that hosts them. The plant takes in carbon dioxide through photosynthesis and feeds it to the fungus.
What the fungus does with the payment
Carbon prompts the fungus to send out hyphae, thread-like filaments a tenth or a twentieth the width of a human hair, according to Justin Stewart of the Society for the Protection of Underground Networks. The threads go deep into the soil and gather nitrogen and phosphorus. Then they carry it back to the plant.
The scale of the return is large. The fungi can supply up to 80 percent of the phosphorus a plant needs and up to 20 percent of its nitrogen. Their hyphae can extend the reach of plant roots by up to 100 times.
The exchange happens inside the root. Plants release chemicals that draw the fungi in, and the fungus builds an arbuscule, which Toby Kiers of Vrije Universiteit Amsterdam describes as a mini tree inside a root cell. That structure is where the trade takes place.
Watching the trade grow
Until recently nobody had tracked how these networks form while they trade. A team built a robot that imaged up to 40 plant and fungus cultures at once and followed more than 500,000 fungal nodes. They also measured around 100,000 flows of cytoplasm inside the hyphae.
The networks grew as travelling waves. Growing tips pulled an expanding front of nutrient-absorbing threads behind them, and fusion between threads kept the density in check. A small carbon investment let the fungus push past the zone where it had already used up the nutrients.
The team measured carbon moving through the networks at up to 120 micrometres a second.
An exchange rate that moves
The BBC Radio 4 programme Naturebang describes the arrangement as a market that is about 400 million years old. In it, nutrients trade for carbon, and the rate between them shifts constantly to match supply and demand.
In June 2026 researchers published a global map in Science. It estimates that the top layer of soil holds about 110 quadrillion kilometres of this fungal network. They also estimate that croplands hold about half the network density of wild ecosystems.
